The mechanisms behind the alteration of chemical carcinogenesis by the dietary lipotropes, choline, methionine, folic acid and vitamin B12 have been studied. The metabolism and carcinogenic activity of ethionine in different species is being compared. Ornithine decarboxylase, a marker of tumor promotion and a precursor enzyme for the polyamines, is used to study the possible role of methyl insufficiency in tumor promotion. The levels of S-adenosylmethionine in the livers of animals treated with hepatocarcinogens and liver tumor promoters are being determined. The results obtained to date are consistent with the hypothesis that methyl insufficiency promotes hepatocarcinogenesis.